In recent years, higher power densities and improved energy densities have been required of electrochemical devices including cells and capacitors. Organic electrolytic solutions have found wider use than aqueous electrolytic solutions from the viewpoint of voltage resistance. Examples of organic electrolytic solutions are those prepared by dissolving alkali metal salts or solid ammonium salts in an organic solvent such as propylene carbonate. Electrolytic solutions of the former type are used for lithium ion battery, while those of the latter type are used for electric double-layer capacitor. Organic electrolytic solutions are inferior to aqueous solutions in electrical conductivity, and numerous studies have been made on organic solvents or electrolytes to obtain improved electrical conductivity.
The electrical conductivity of nonaqueous electrolytic solutions comprising such a solid electrolyte dissolved in a solvent varies with the concentration of the electrolyte. With a rise in the concentration, the ion concentration of the solution increases to increase the electrical conductivity, which will reach a maximum in due course. The electrical conductivity reaching the maximum starts to decrease presumably because the electrolyte becomes difficult to dissociate and increases in viscosity at the same time owing to increased interaction between the solvent and ions and between the ions as the number of ions increases in the electrolytic solution. When further increasing in concentration, the electrolyte becomes no longer dissociable, and the concentration of the electrolyte is saturated. Thus, an attempt to increase the concentration of the electrolyte encounters the problem that the electrolyte becomes less soluble. Another problem is also experienced in that when electrolytic solutions having an electrolyte dissolved therein at a high concentration is used in an environment of low temperature, a salt will separate out to impair the electrical conductivity of the solution.
Found in recent years are salts having a melting point around room temperature or salts having a melting point not higher than room temperature (salts melting at room temperature). It is known that even if solid at room temperature, such salts dissolve in organic solvents at a higher concentration than usual electrolytes. Furthermore, the salts melting at room temperature are miscible with a specific organic solvent in a desired ratio. Accordingly, these salts afford electrolytic solutions having a high concentration not available by dissolving conventional solid electrolytes in organic solvents, while although having a high concentration, the solution is less likely to encounter the problem that the salt will separate out in a low-temperature environment. The salt melting at room temperature is itself liquid and is therefore usable singly as an electrolyte.
It is disclosed that aliphatic ammonium salts having an alkoxyalkyl group introduced thereinto are highly soluble in a nonaqueous organic solvent and are less likely to separate out at low temperatures (patent literature 1). However, satisfactory improvements still remain to be made in the electric conductivity of ionic liquids per se, or in the electric conductivity of electrolytic solutions as diluted with an organic solvent.
Many of anions of these electrolytic solutions contain fluorine, chlorine or like halogen. Many halogen-containing anions have high electric conductivity and high voltage resistance but place a heavy burden on the environment. Fluoric anions especially include many which have the drawback of hydrolyzing in the presence of water to produce highly toxic hydrogen fluoride.    [patent literature 1] WO 02/076924
An object of the present invention is to provide an electrolyte which is highly soluble in organic solvents, highly reliable at low temperatures, highly electrically conductive and less likely to burden the environment, and further to provide electrochemical device having such advantages.